The injection of the small amount of diphtheria toxin used in the Schick test may act as a secondary stimulus.

A Schick test may therefore cause a great and rapid increase in the immunity of the animals tested.

Examples are quoted of six rabbits and twelve guinea-pigs in Tables II to VII.

2. A fraction of a Schick dose may act as a secondary stimulus. Rabbit G 23 quoted in Table IV, injected with 1/10 of a Schick dose, showed an immunity response, the antitoxic content of its blood rising from 1/50 to nearly 1/10 unit per c.c. in six days.

3. The action of a Schick dose as a secondary stimulus may cause an animal to give a negative reaction when tested seven days or more after the first positive reaction.

This is illustrated by rabbits G 7 in Table III, G 31 in Table II, G 32 and G 34 in Table V and four guinea-pigs in Table VI.

4. The antigenic value of a Schick dose of toxin as a secondary stimulus may be as high as that of a reasonable dose of a toxin-antitoxin mixture suitable for human immunisation. Examples are given comparing the results of the injection of a Schick dose of toxin and of a toxin-antitoxin mixture in the same rabbit in Table III, in different rabbits, G 20 and G 22 in Table IV and reference is made to the companion rabbits to those quoted in Table V.

5. The antigenic value of a Schick dose as a secondary stimulus can be demonstrated:

A. In animals which have not produced a detectable quantity of antitoxin (that is less than 1/2000 of a unit per c.c.) as the result of a primary stimulus.

See both rabbits in Table II, rabbit G 20 in Table IV, both rabbits in Table V, and guinea-pig FF 19. v in Table VII. The four guinea-pigs in Table VI probably come under the same heading.

B. In animals whose actively produced antitoxin has fallen below a de tectable level.

See rabbit G 7 in Table III.

(These results add further confirmation to the phenomenon reported in the paper “Active immunity to diphtheria in the absence of detectable antitoxin” (Glenny and Allen, 1922).

6. A Schick dose of toxin which gives a positive reaction may, by acting as a secondary stimulus, produce a rapid increase in the antitoxic value of animals already containing some actively produced antitoxin.

See guinea-pig LL 17. vi in Table VII.

7. A Schick dose of toxin which causes no reaction may, by acting as a secondary stimulus, produce a rapid increase in the antitoxic value of animals already containing some actively produced antitoxin.

See guinea-pigs in Table VII.

8. A Schick dose of toxin may fail as a secondary stimulus if the antitoxic content at the time of injection is comparatively high.

See rabbit G 21 in Table IV.